In the field of optical inspection of objects such as semiconductor wafers, reticles, flat-panel displays, and other devices, the particular illumination source that is used can greatly affect the inspection results.
For example, illumination may be characterized as narrowband illumination or broadband illumination in the wavelength domain. Illumination may be further characterized as long-pulsed (i.e. long pulse in the time domain) or short-pulsed (i.e. short pulse in the time domain). Together, the characteristics are shown in FIG. 12, which also shows where some exemplary sources generally fall in the characterizations.
Some inspection systems use lamps, some systems use continuous lasers, and some systems use short-pulsed lasers. Inspection systems that utilize lamps may select narrowband or broadband illumination. However, lamps generally must be continuously illuminated or long-pulsed in the time domain. Although lamps may provide options with regard to the type of illumination, when high power illumination is required, short-pulsed lasers may be preferred. For instance, short pulsed lasers may be especially relevant for 2-D imaging. Furthermore, short-pulsed lasers may be advantageous in that the short illumination time reduces errors caused by pixel smear. However, presently-existing inspection systems that utilize short-pulsed lasers may suffer from certain disadvantages.
While lamps, as noted above, may be used for broadband inspection, presently-existing systems that utilize pulsed lasers provide for a relatively narrow bandwidth of illumination. This may lead to problems in the inspection. One such problem is “color variation.” For instance, differences in the thickness of layers in a semiconductor wafer may lead to differences in the colors and/or intensity of layers. In monochromatic imaging, the differences may appear as different intensities, while in polychromatic imaging, the differences may appear as different colors. In any event, any such intensity/color differences can trigger false alarms even if the changes in thickness in the wafer are within tolerated ranges. Another problem with narrowband laser illumination is speckle caused by interference between coherent illuminating photons. Various schemes have been proposed for reducing speckle through additional optical components. However, the use of such components may deteriorate the illumination quality and/or its intensity and result in a degraded image.
Accordingly, a need remains for an inspection system that can provide improved inspection results.